powerpc/xive: Add missing null check after calling kmalloc

Commit 930914b7 ("powerpc/xive: Add a debugfs file to dump
internal XIVE state") forgot to add a null check.

Add it.

Fixes: 930914b7 ("powerpc/xive: Add a debugfs file to dump internal XIVE state")
Signed-off-by: Ammar Faizi <ammarfaizi2@gmail.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211226135314.251221-1-ammar.faizi@intel.com